As is known, disc brake assemblies, for instance used on land vehicles, incorporate brake pads in which a layer of friction material is provided.
Conventionally, brake pads are made starting from a metal back plate on which a surface adapted to receive the friction material is defined.
Back plates for brake pads usually have irregular shapes and their size in the plane of the back plate, in longitudinal direction, i.e. in the direction tangent to the trajectory of the circular motion of the disc to which the back plates are to be applied, is greater than the size in transverse direction, i.e. in the direction perpendicular to said longitudinal direction.
The friction material is applied to the surface of the back plate at a predefined engaging portion which usually takes up most of the area available on said surface. This engaging portion is also usually surrounded, without interruption, by a free peripheral zone, i.e. a zone intended not to be covered by the friction material.
The peripheral zone, free from the friction material, is necessary mainly for working requirements. The width of the free zone is usually kept to the bare minimum, as it is disadvantageous, mainly for reasons of weight, that the surface of the back plate to which the friction material is applied has an area bigger than the area effective for the braking effect.
The minimum width of said free zone, usually measured along the longitudinal flanks of the back plate, can vary from some tenths of millimeter to some millimeters.
During operation of the brake assembly, i.e. during braking, the brake pad is brought against the disc along a direction substantially perpendicular to the surface of the rotating disc. During braking the layer of friction material with which the brake pad is provided is exposed to forces that are mainly tangential with respect to the circular motion of the disc, said forces developing in the plane of the brake pad parallel to the disc and tending to remove the friction material from the back plate. In addition to these forces, there are also other forces that are directed in several directions, particularly perpendicularly to the surface of the brake pad, and are caused by the vibrations generated during action of the brake pad on the disc and as a consequence of possible alignment errors of the parts in relative motion. Furthermore, part of the kinetic energy of the disc turns into heat during braking. The heat generated during braking usually causes an increase in the temperature of the materials of which the brake pad is made. The increase in temperature may cause weakening of the bonds that hold the friction material on the back plate.
The more the braking action to be exerted by the brake assembly onto the rotating disc is, the stronger the forces acting on the friction material during braking are. When such forces overcome the counter-forces holding the fiction material adhering to the back plate, detachment and destruction of the friction material or of part thereof occur, thus causing jeopardy to or loss of the braking effect.
The back plate, the friction material and the method for their manufacturing therefore have to be devised by taking into account the stresses to which the brake pad is exposed when in use.
In order to increase adhesion of the friction material to the traditionally smooth surface of the back plate, back plates were proposed in the past which are provided with retaining elements, each of said elements consisting of a groove and a corresponding protrusion integrated on the surface of the back plate having to receive the friction material.
According to prior art, the protrusions are obtained by making grooves in the material of the surface of the back plate and lifting from said surface the material removed from the groove, without, however, removing it entirely from the body of the back plate.
With this technique back plates for brake pads are nowadays manufactured which are provided with a plurality of protrusions of different shape and size, for instance curl-shaped protrusions.
U.S. Pat. Nos. 7,222,701 and 7,249,483 describe examples of back plates provided with retaining elements of the aforementioned kind.
The methods currently used for manufacturing retaining elements of the kind provided with grooves and engagement protrusions on the surface of the back plates, however, do not allow to obtain an optimal distribution of the retaining elements. It may happen, for instance, that too large areas of the surface of the back plate to which the friction material is to be applied remain free of retaining elements.
It is therefore a first object of the invention to overcome this drawback by providing a back plate in which the distribution of grooves and protrusions is optimized as a function of the forces applied to the brake pad when the latter acts against the disc of a brake assembly for stopping rotation thereof.
Retaining element are currently distributed mostly randomly over the surface of the back plate, often by giving priority to working requirements rather than to the requirements of the products to be obtained. This disadvantageous condition occurs in particular in the proximity of the perimeter delimiting the engaging portion, defined on the surface of the back plate, which is intended for receiving the friction material.
Nowadays retaining elements are made on the surface of the back plate, usually by means of a mechanized equipment having a plurality of parallel tools provided with teeth or cutting edges. The tools cut the material of the back plate thus creating corresponding grooves. The material lifted by the tools during the making of the grooves is brought over the plane of the surface of the back plate until it forms as many engagement protrusions, typically in the form of curls or shavings.
A first kind of known equipment provides that the tools are arranged mutually parallel and act on the back plate by approaching the back plate all from the same lateral flank. It is clear that in this case the opposite lateral flank of the back plate will have to be kept firmly in abutment against a fixed counter-surface. This method has the drawback that forces of high intensity are applied along the flank of the back plate opposite to the advance direction of the tools. The lateral flanks of the back plates follow a usually irregular contour that may often include a series of rectilinear sections, variously inclined sections, curved sections, concave or convex sections. Therefore clearances may be easily created between the back plate and the abutment surface of the equipment used for making the retaining elements. These clearances are further accentuated by the wear of the equipment. Because of these circumstances, in addition to possible errors of alignment of the back plate, the forces applied during cutting may deform the back plate flank which is in abutment against the counter-surface. The obtained back plate may therefore sometimes have a quality that is insufficient and must therefore be discarded.
A second kind of equipment provides tools arranged mutually parallel which act simultaneously by approaching the back plate from opposite flanks. Usually tools of even order approach from one flank and those of odd order approach from the opposite flank. This arrangement makes it unnecessary to provide a counter-surface. This method ensures a better result than the first described method. The forces applied to the surface of the back plate by a first order of tools, for instance tools of even order, are indeed counteracted by the tools of the other order, of the odd order in the example, and therefore there are no deformations of the peripheral flanks of the back plate. However, this second manufacturing method, too, is not free from drawbacks.
First of all, this kind of manufacturing, because of the curvilinear or broken, and in any case usually irregular contour of the lateral flanks of the back plate, does not allow to easily make retaining elements in the proximity of the perimeter of the back plate. On the other hand, it is in any case necessary to avoid protrusions or grooves so close to the perimeter that they are only partially embedded in the friction material or are even outside it. This circumstance would indeed create a zone of weakness for the adhesion of the friction material to the back plate and must therefore be avoided. For the aforesaid reasons the currently manufactured back plates usually have a usually continuous peripheral zone provided within the perimeter of the engaging portion and delimited towards the outside by said perimeter, in which zone there is no optimal distribution of the retaining elements. In the back plates of the prior art, the width of said zone is often too great, at least in some parts of the back plate, and in this case it may cause breaking and detachment of the friction material.
It is therefore a further object of the invention to provide a method for obtaining an optimal distribution of the retaining elements even in the proximity of the perimeter of the engaging portion defined on the engagement surface of the back plate.
A not least object of the invention is to provide a back plate and a method for manufacturing the same that can be obtained at low costs and are therefore suitable for large-scale industrial production.